[Strong and tunable field enhancement obtained by periodic rectangular pit structure].

The authors have designed a novel type of periodic rectangular pit nanostructure substrate based on the surface plasmon principle. Finite element method was employed to simulate the optical near-field distribution. Strongly enhanced field whose electric intensity Emax/E0 can be as high as 20 at resonance frequency appears around pithead of the periodic structure. As the period of structure, pit length l, width w and environment change, the authors observe the regular shifting of plasmon resonant wavelength which can cover the range from 500 to 1000 nm. The red shifts of SPR resonance peaks are increased with the increment of period Px when incident light is polarized along x axis. An abrupt decrease in localized electric field in the pit is observed as incident wavelength approaches Px. This is due to the satisfaction of wave vector matching condition and the excitation of propagating SPP. SPR resonance peaks also red shifts with the increment on pit length l and environment dielectric refractive index, presenting a linear dependence with pit length l. While the resonance peaks are blue shifted with the increment of pit width w. The results presented in this paper will provide a way to tune the plasmon resonant wavelength. Inspired by Jain's report, SPR resonance peaks' shifting with the changing of structure parameters can be explained by viewing the rectangular pit nanostructure as combination of two pairs of dipole-dipole coupling models along x and y axis respectively.